1. Field of the Invention
The present invention relates to a composite metal hydroxide, a flame retardant containing the composite metal hydroxide which is capable of exhibiting excellent flame retardancy and acid resistance when incorporated into resins and rubbers (to be sometimes simply referred to as "resin" or "resins" hereinafter), and a flame-retardant resin and/or rubber composition (to be sometimes simply referred to as "resin composition" hereinafter) containing the flame retardant. More specifically, the present invention relates to a halogen-free composite metal hydroxide capable of imparting a resin with excellent flame retardancy and acid resistance, a flame retardant containing, as an active component, the composite metal hydroxide and being free from a foaming trouble of a molded article, which occurs due to the decomposition of a flame retardant at a resin-processing temperature, and a flame-retardant resin composition containing the flame retardant.
2. Prior Art of the Invention
In recent years, the flame retardancy of a resin has been increasingly demanded, and the severity of the demand is increasing. To satisfy such an increasing demand for flame retardancy, a flame retardant containing an organic halide and antimony trioxide in combination has been conventionally proposed and used. However, such flame retardant has the following defects: When a composition containing this flame retardant is processed, the flame retardant is partially decomposed to generate a halogen gas. As a result, the processing and molding machines are corroded. Further, this flame retardant has toxicity to workers, has a harmful influence on the heat resistance and weatherability of a resin, and generates a large amount of smoke containing a toxic gas during the combustion.
It has been therefore increasingly demanded to develop a halogen-free flame retardant which is free from the above defects, and aluminum hydroxide and magnesium hydroxide have attracted attention. However, aluminum hydroxide starts dehydration at about 190.degree. C. to cause a foaming trouble of a molded article. Therefore, the molding temperature is required to be kept below 190.degree. C., and the kind of resins to which aluminum hydroxide can be applied is limited.
Magnesium hydroxide starts dehydration at about 340.degree. C., and therefore has an advantage in that it can be applied to almost all resins. Further, a process for synthesizing a magnesium hydroxide, by which a well-grown crystal of magnesium hydroxide can be produced, has been developed by inventors including the present inventor. As a result, excellent molded articles can be now obtained, and compositions containing such magnesium hydroxide are applied to communication cables, ships, and the like.
However, the above magnesium hydroxide has now been found to still have new problems to overcome. The first problem is that the above magnesium hydroxide exhibits its high-level flame retardancy only when a relatively large amount of it is incorporated into a resin. For example, when the above magnesium hydroxide is incorporated into polypropylene, it is required to incorporate about 150 to 250 parts by weight, per 100 parts by weight of polypropylene, of the above magnesium hydroxide in order to satisfy V-0 (highest flame retardancy level) of UL-94 flame retardancy standard at a thickness of 1/8 inch to 1/16 inch. When such a large amount of magnesium hydroxide is incorporated, the following problem arises: Out of the physical properties inherent to a resin, the Izod impact strength, elongation and tensile strength decrease.
The second problem is that a resin containing magnesium hydroxide is poor in acid resistance. For example, a molded article such as communication cable or a power cable produced from a resin composition containing polyethylene and magnesium hydroxide is in contact with water for a long period of time, magnesium hydroxide is gradually dissolved in carbonic acid contained in water and migrates toward the molded article surface to precipitate magnesium carbonate. As a result, the surface is whitened. The flame retardancy decreases in the long run by a proportion of magnesium hydroxide that has been dissolved and converted to magnesium carbonate. Further, the electric insulation of the above cables decreases.